Fixed typo

Corrected spelling of the NUL character (was spelt "null").
Added missing `require'

elwiki-test did not `require' ert.
Added elwiki-vc test and input validation

* `elwiki/log->alist' now throws an error when `commit-log' does not
  contain 4 fields.

* Added an ERT file for elwiki-vc; only has a test for
  `elwiki/log->alist'.